The Abyssal Plague

Tharizdun, the Chained God, has released the Voidharrow--a liquid crystal imbued with his will and all the chaos of the Abyss--into the universe, and with it comes a transformative engine of pestilence. When exposed to the Voidharrow, individuals look as though they've been stricken by disease as the Abyssal liquid twists and deforms them into chaotic demonlike creatures. Behold the Abyssal Plague! - Under the Crimson Sun

Compared to other threats on Athas, most afflicted by the Abyssal Plague could easily be mistaken for other threats on Athas. Wasteland mutants are not uncommon and even some areas of Tablelands have been known to warp normal creatures into monstrousities.